Waveguide-stripline transducer



Aug.19,1969 RHMNERR 3,462,713

4 WAVEGUIDESTRIPLINE TRNSDUCER I Filed July 19, 1967 5 Sheets-Sheet 1 ATTORNEY Aug. 19, 1969 R. H. KNERR wAvEGUIDE-STRIPLINEA TRANSDUCER FiledJuly 19, 1967 5 Sheets-Sheet 2 FIG. 2B

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Aug. 19, 1969 R.H. KNERR 3,462,713

WVEGUIDE-STRPLINE TRANSDUCER l Filed July 19. 1967 r 5 sheets-sheet sUnited States Patent O1 ice 3,462,713 WAVEGUIDE-STRIPLINE TRANSDUCERReinhard H. Knerr, Bethlehem, Pa., assignor to Bell TelephoneLaboratories, Incorporated, Murray Hill, NJ., a corporation of New YorkFiled July 19, 1967, Ser. No. 654,480 Int. Cl. H0111 1/16 U.S. Cl.333-21 5 Claims ABSTRACT F THE DISCLOSURE A broadband waveguide tostripline transducer in which the end of the stripline center conductoris directly inserted into the waveguide and is provided with lumpedreactive elements which form a plurality of stagger tuned resonantcircuits with distributed reactances of the inserted portion.

BACKGROUND OF THE INVENTION This invention relates to radio frequencytransducers and more particularly to a transducer for coupling togethera radio frequency waveguide of the hollow conductor type and one of theparallel strip, multiple conductor type, such as the stripline orMicrostrip.

Mixed microwave circuits, in which part of the circuit is inconductively bounded waveguide and part is in the form of parallel stripconductors, are becoming increasingly popular with the development ofintegrated circuit techniques. In these circuits it is generallynecessary to transfer energy one or more times between guides of thesedifferent types. One form of transducer now being used in the art forthis purpose employs a short sectional coaxial line matched at one endto the stripline and having the center conductor thereof at the otherend forming a probe which extends into and is matched to the waveguide.Examples of transitions between stripline and coaxial conductors may befound, for example, in United States Patent 2,794,174 granted May 28,1957 to M. Arditi et al. or 3,201,721 granted Apr. 17, 1965 to R. W.Voelcker. Transitions between coaxial lines and waveguides are shown inpublications by S. B. Cohn in the September 1947 Proceedings of theI.R.E. at p. 920 or by W. W. Mumford in the February 1953 Proceedings ofthe I.R.E. at p. 256. This combined transducer is, however, cumbersomeand expensive.

If the center conductor of the stripline itself could be directlyinserted into the waveguide in a manner analogous to the coaxial probe,the transition could be physically simplified. This approach has notheretofore proved satisfactory, except for very narrow bandwidths,because the stripline center conductor so inserted forms with thewaveguide a sharply resonant circuit at some frequency. Unless thisfrequency falls within the band 0f interest there will be insufficientcoupling, but if the frequency can be adjusted to fall within the band,its sharp resonance will severely limit the band of coupled energy.

SUMMARY In accordance with the invention, the portion of the centerconductor directly inserted into the waveguide is provided with apattern of substantially lumped reactive elements, at least one of whichis within the guide, the others, if any, being close to the guide. Whenthese reactive elements are tuned with the distributed reactance of thestripline center conductor to a plurality of spaced individualfrequencies within the band, the resulting series of stagger tunedcircuits produces a broadband match. Each reactive element may be eitherin the form of a capacitive crosspiece or stub with respect to thecenter conductor, or altematvely, an inductive notch in the 3,462,713Patented Aug. 19, 1969 center conductor. Both stubs and notches havebeen used in the art as reactances to modify the characteristicimpedance of the strip for impedance matching purposes and it is knownthat they can be substituted one for the other if accompanied byphysical relocation of one-quarter wavelength along the length of astripline.

BRIEF DESCRIPTION OF DRAWING FIG. l is a cutaway perspective view of awaveguidestripline transducer in accordance with the invention;

FIG. 2a is a cross sectional view taken through FIG. 1 as indicated toshow the stripline pattern in accordance with the invention;

FIG. 2b is the approximate equivalent circuit of the pattern of FIG. 2a;

FIG. 3 is a typical coupling versus frequency characteristic for theembodiment of FIGS. l and 2a; and

FIGS. 4a through 6a are alternative stripline patterns together withtheir approximate equivalent circuits in FIGS. 4b through 6brespectively.

DETAILED DESCRIPTION Referring more particularly to FIG. 1, anillustrative embodiment of a transducer is shown which provides couplingbetween a rectangular conductively bounded waveguide 10 and a parallelconductor stripline 11. For illustration, line 11 is specifically shownas the symmetrical type having a thin center conductor or strip 12interposed between a pair of ground planes which are formed in theparticular embodiment illustrated by the wider inside surfaces of achannel cut in body 13. Strip conductor 12 is typically supported inthis channel by being formed of conductive material plated or printedupon a supporting substrate 14 of high dielectric material. lIt shouldlbe understood, however, that the invention may be applied to lines ofself-supporting center conductor or to the unsymmetrical type of line,sometimes referred to as the Microstrip, in which a first thin conductoris related to only one ground plane. Furthermore, the ground planes maybe printed or plated surfaces in the familiar sandwich construction.

The coupling between guide 10 and stripline 11 is formed by extendingcenter conductor 12 through an elongated aperture 15 in the wide wall ofguide 10` which aperture preferably has the same cross sectionaldimensions as and is aligned with the channel in body 13. Thus theextended center conductor forms a strip shaped probe 16 within the guideparallel to the undisturbed electric field polarization in guide 10. Theposition of aperture 15 is displaced away form the longitudinal centerline of guide 10 in accordance with known impedance matchingconsiderations and the end of guide 10 is closed by a short circuit,piston or conductive transverse wall 19 spaced from the position of theprobe 16. Details concerning the location of both the probe and theconductive short are disclosed, for example, in terms of coaxial probesin the above-mentioned article The Optimum Piston Position for Wide-BandCoaxial-to- Waveguide Transducer by W. W. Mumford, appearing in theFebruary 1953 Proceedings of the I.R.E. at p. 256. It has beendetermined that it is immaterial whether the plane of strip shaped probe16 is parallel to a narrow wall of guide 10 or parallel to wall 19.

It should now be noted that the stripline ground planes which as to theupper portion of strip 12 comprised the surfaces of the channel in body13, are now effectively replaced as to probe portion 16 by the walls ofguide 10.

In accordance with the invention it has been found thatV condition inthe band of interest. An equivalent circuit will be describedhereinafter in connection with FIG. 2 which attempts to identify thecomponents of distributed reactance which porduce the observed conditionof resonance. Unless this resonant frequency falls within the band ofinterest there is limited coupling and if the resonance does fall withinthe band, the coupled band is very narrow.

It is known in the art that stripline stubs extending on one or bothsides of the center conductor constitute capacitive reactances, as Shownfor example, in the above-mentioned Arditi patent. In accordance withthe present invention the coupled bandwidth is broadened by stubs 17 and18 of this type placed, however, upon the probe 16 portion of conductor12 within guide 10. In particular stubs 17 and 18 have such dimensionsand locations on probe 16 that they form with the distributed reactancesof probe 16 within the guide, a second resonance at a frequency withinthe band of interest but adjacent to and different from the resonantfrequency produced as a result of the distributed reactances of probe16. The interaction of these two resonant circuits has the same effectas stagger tuned circuits used in filters or equalizers, for example,and broadens the coupled bandwidth as will be shown in connection withFIG. 3.

FIG. 2a is -a cross sectional view taken through guide in front ofdielectric support 14 and shows the stripline center conductor 12forming probe 16 as it extends within guide 10. An equivalent circuit isshown in FIG. 2b having inductances L and certain capacitances C and Carepresenting the distributed reactances of that portion of strip 12within guide 10. Outside of guide 10 reactances corresponding only to Land C are present, and customary design usually results in a netinductive reactance and nonresonant condition at the frequency ofinterest. Within the guide, however, capacitance Ca represents thatportion of the total distribtued capacity that is in series with thecenter conductor due to the electric field component in guide 10parallel to probe 16 of strip 12 which depends in turn upon the distancea of FIG. 2a. It is this capacity which produces the band limitingeffect to which the present invention is directed. The capacity C thenrepresents the remaining small distributed capacity due to the electriceld perpendicular to the strip. Stubs 17 and 18 produce a substantiallylumped capacity represented on the equivalent circuit of FIG. 2b by Cb,large compared to C, and dependent upon the dimension b of FIG. 2a aswell as upon the width of stubs 17 and 18. When the circuit loopsinvloving Ca and Cb, as indicated in FIG. 2b, are tuned to differentfrequencies f1 and f2 within the band, improved coupling in accordancewith the invention is achieved as is shown in FIG. 3 by the familiardouble humped coupling versus frequency characteristic of stagger tunedcircuits. The spacing Af between the respective hump center freqeunciesf1 and f2 is readily adjusted by control of the spacing of stubs 17 and18 from the end of probe 16. This spacing determines the amount ofdistributed inductance L along with a smaller amount of distributedcapacitance C included within each circuit loop. Obviously, in apractical embodiment the parameters are not as discrete as theequivalent circuit seems to indicate. Furthermore, since other variablesinclude 'the position of the short, the length of the probe and itsposition, proper proportions of the center conductor pattern are bestdetermined on an empirical basis after an approximate choice of theother parameters on the basis of known coaxial probe relationships.

One or more additional capacitive reactances, further producing one ormore separate resonant circuits, may be added to further increase thebandwidth. Thus in FIG. 4a stubs 21 and 22 are added which havedimensions different from those of stubs 17 and 18 and are spacedtherefrom along center conductor strip 12. The dimension c of stubs 21and 22 determines a capacity Cc as shown on FIG. 4b and the spacing ofstubs 21 and 22 from stubs 4 17 and 18 determines the value of L and Cas required to produce resonance at a new frequency f3.

Notches such as 23 and 24 of FIG. 5a reduce the width of centerconductor 12 and introduce an inductance in series with the line asindicated by inductance Ld of FIG. 5b. In general a notch is equivalentto a stub located one-quarter Wavelength further along the line from theposition of the stub which is replaces. Thus a plurality of notches 25as shown in FIG. 6a and having the equivalent circuit of FIG. 6b cansimilarly be employed.

It should be understood that the added reactances can only produce theimprovement in accordance with the invention if they tune with thedistributed reactance of probe 16 within the guide. Otherwise addedreactance only serves to modify the characteristic impedance of the linein accordance with prior art teachings, and only incidentally, if atall, to effect bandwidth of any coupling means at the end of the strip.It has been found, however, that if at least one of the added reactancesis within the guide, additional improvement may be obtained from othersnot physically within the guide but which are relatively close thereto.It is believed that improvement in accordance with the invention can beobtained so long as the added reactance is in the transition regionwhere field patterns are changing from those of the waveguide mode tothose of the stripline mode. It is further believed that a reactancemore than one-quarter wavelength away from the guide can have littledirect effect upon the coupled bandwidth.

While in the particular form described, the plane of the stripline wasshown inserted parallel to the narrow side of the guide, it should benoted that any given pattern may be turned ninety degrees in the guideso that its plane is perpendicular to the narrow side without effectingthe coupled bandwidth. This versatility is one of the primary advantagesof the coupling in accordance with the invention. In certainexperimental models it was, however, found necessary to make slightreadjustment of the shorting piston position when the pattern isrotated.

In all cases it is to be understood that the abovedescribed arrangementsare merely illustrative of a small number of the many possibleapplications of the principles of the invention. Numerous and variedother arrangements in accordance with these principles may readily bedevised by those skilled in the art without departing from the spiritand scope of the invention.

What is claimed is:

1. A combination in which a hollow conductively bounded waveguide iscoupled for frequencies of electromagnetic wave energy within a givenwide band to a second waveguide of the type having a strip conductordisposed in parallel spaced relationship to at least one conductiveground plane, said coupling being produced by extending a portion ofsaid strip conductor into said hollow waveguide, means for introducing afirst substantially lumped reactance to a first portion of said stripwithin said guide, characterized in that said lumped reactance is tunedwith a portion of the distributed reactances of said portion toresonance at a first frequency within said given band and furthercharacterized in that means are provided for introducing a secondsubstantially lumped reactance to a second portion of said stripdisplaced along the length thereof from said first portion but incoupling relationship to fields of wave energy in said hollow waveguide,said second reactance being tuned to resonance within said band at afrequency slightly different from said first frequency to broaden thetransmission band of electromagnetic energy coupled between saidwaveguides.

2. The combination according to claim 1 wherein one of said lumpedreactances comprises a conductive crosspiece upon said strip within saidhollow guide and the other comprises a notch in said strip.

3. The combination according to claim 1 wherein one of said lumpedreactances comprises at least one conductive stub upon said strip withinsaid guide.

5 6 `4. The combination -according to claim 1 wherein one OTHERREFERENCES of said lumped reactances comprises at least one notchParallel-Plate Transmission Systems for Mirowave Frequencies-A. F.Harvey--The Proceedings of the Institution of Electrical Engineers,London (Part B No.

5 26) March 1959 (vol. 106) pp. 129-133.

in said strip within said guide.

5. The combination according to claim 1 wherein said lumped reactancescomprises conductive crosspieces of diierent size pon said strip withinsaid guide.

References Cited HERMAN KARL SAALBACH, Primary Examiner UNITED STATESPATENTS MARVIN NUSSBAUM, Assistant Examiner 2,829,348 4/1958 Kosrrtza etal. 333-84 XR 10 U5 C1. XR. 2,877,426 3/1959 KostllZa et al. 333-84 XR2,884,601 4/1959 KostriZa et al. 333-84 XR 3331-84, 98 2,979,676 4/1961Rueger 333--34 3,265,995 8/1966 Hamasaki 333-21

